Anti-skid brake system



Feb. 24, 1959 D. E. BROWN 2,874,810

' ANTI-SKID BRAKE SYSTEM Filed April 2, 1956 2 Sheets-Sheet 1 Feb. 24,1959 D. E. BROWN ANTI-SKID BRAKE SYSTEM Filed April 2, 1956 mw wwwwwmwwwwww INVENTOR.

. y@ J. @eA/Ey United States Patent O ANTI-SKID BRAKE SYSTEM DwightEugene Brown, Sherman Oaks, Calif., assignor to Hydro-Aire, Inc.,Burbank, Calif., a corporation of California Application April 2, 1956,Serial No. 575,524

17 Claims. (Cl. 18S- 181) This invention relates to a declerationcontrol system for aircraft, and more particularly to brake controlmeans for preventing or minimizing the sliding and slipping of thelanding wheels during application of the brakes.

It isl well recognized that the most efficient and etfective method ofbraking a wheeled vehicle is to apply the maximum friction drag to thewheels without causing the tires to slip or skid on the supportingsurface. In practice the achievement of ideal braking automatically hasbeen diiicult if not impossible since the correct braking force to thevehicle wheels must vary with the rotational speed and the coefficientof friction between the wheels and the runway. The coeicient of frictionvaries widely depending on construction materials, temperature andweather conditions. The coefficient of friction also has an importanteffect on the time required for a nonrotating wheel, as well as anexcessively braked wheel rotating at less than the linear speed of thevehicle, to reaccelerate into synchronism with the linear speed of theaircraft. A rotational wheel speed corresponding with the linearvelocity of the craft is often called the synchronous speed of thewheel. Since the tire is somewhat elastic, it will deect and slip at theleading and trailing edges of its footprint under severe brakingconditions. For this reason, synchronous speed as used in thisapplication will be understood to be the wheel speed at which the majorportion of the tire footprint does not slip with respectto thesupporting surface as determined by the maximum coefficient developed.It is not necessarily the same as that calculated from the rollingradius of the tire. If an aireld runway is wet or icy, a wheel caneasily be locked by application of the brake due to the low coefficientof friction; for the same reason, a locked wheel will require aconsiderably longer period to accelerate lto synchronous speed afterrelease of the brake than under dry runwayv conditions. The timerequired for a wheel to recover or accelerate to synchronousspeed afterexcessive braking action is of considerable importance since the brakes.are most etfective when applied while the wheel is rotating atsynchronous speed.

Another feature of importance is the provision of automatic means toassure release of the brakes so long as the landing wheels are out ofcontact with the ground while airborne or while bouncing in touchingdown on the landing field. The reason for this is that it might bedisastrous for a plane to land with locked wheels. And it might beequally serious if the brakes could be applied while the plane is inmid-air during a bounce from the runway.

A further feature of a satisfactory brake control system is theprovision of means for deactivating the automatic controls in favor ofmanual brake control after the craft has been braked to a safe speed forpilot control of the brakes while taxiing and parking the plane.

ln view of the above-mentioned desirable characteris- 2,874,810 PatentedFeb. 24, 1959 ice tics of a satisfactory brake control system forregulating7 the deceleration of vehicle wheels, it is an object of thepresent invention to provide a highly effective and compact automaticsystem for minimizing the slipping and skidding of a tire during brakingto achieve the most eicient, eifective and safe braking of the vehicle.

A further object is to provide an automatic brake control system havingan improved electrical sensing circuit for sensing the rate ofdeceleration of each braked wheel from its synchronous speed upon brakeapplication and to delay reapplication of the brake until the wheel hasreaccelerated to synchronous speed.

A further object is to provide a brake control system in whichdifferentially rated capacitors are employed to measure wheeldeceleration and reacceleration rates in combination with means forselectively utilizing these measurements to effect release of the brakespending recovery of the wheel speed as governed by friction conditionsbetween the wheel and the ground.

A further object of the invention is to provide means for insuring thatthe aircraft brakes are released during iiight without interfering withthe availability of the brakes for use by the pilot during normaltaxiing and parking operations.

Yet another object is the provision of automatic brake control meansemploying a separate sensing circuit coupled to each wheel and makinguse of a multiple coil arming relay having a coil thereof in eachsensing circuit and designed to arm or disarm the brake release circuitdepending upon whether the sum of all of the wheel speeds isrespectively above or below a predetermined speed.

Still another object is the provision of an automatic brake controlhaving means for sensing excessive rates of wheel deceleration andeffecting release of the brake for an initial period, and recovery ratesensing means for continuing the brake release period for a furtherinterval where the recovery speed at the end of the initial period istoo low for safe reapplication of the brake.

Another object is the provision of an automatic brake control systemhaving means for storing electrical energy in an amount proportional towheel speed prior to braking, or during normal braking, and utilizingthis stored energy in response to excessive deceleration of the wheel toeffect release of the brake until the wheel has recovered substantiallyto synchronous speed.

More particularly it is an object to provide an automatic brake controlhaving a sensing circuit adapted to be energized to a potentialproportional to wheel speed together with means for releasing the brakeafter excessive application thereof by use of energy stored in thesensing circuit until the rising potential level in the sensing circuitindicates the wheel speed has recovered.

A further object is to provide circuit means responsive to the speed ofmore than one supporting wheel to eifect A brake release incorresponding pairs on opposite sides of the aircraft when one wheel isdecelerating in excess of normal airplane deceleration or is lockedagainst rotation, thereby preventing conditions conducive to groundlooping.

These and other objects will become apparent from the following detaileddescription of an illustrative embodiment of the invention taken inconnection with the accompanying drawings, wherein:

Figures la and lb cooperatively constitute a schematic diagram of thehydraulic and electrical control portions of the braking systemconstructed in accordance with the present invention, the verticallyarranged terminal strip to the right of Figure 1a being identical withthe strip to the left of Figure lb for convenience in tracing a circuitbetween the two figures.

While the braking control system of this invention may be applied tovarious types of vehicles and to any number of wheels, it will bedescribed below as applied to an airplane with two braked wheels.

In the present specification and claims the term synchronous speedfisemployed to mean a rotational wheel speed corresponding to the optimumbraking speed at which the major portion of the tire footprint does notslip with respect to the supporting surface as determined by the maximumcoefficient of friction developed. The term slip or slipping designatesa condition in which a wheel is rotating at other than synchronousspeed, and is to be distinguished from the term "s'kid or skidding usedto designate a wheel locked against any rotation by the brakes. The termrecovery rate denotes the rate of rotative acceleration followingrelease of the brakes while the vehicle is in motion.

Essentially vthe system to be described comprises a rotating commutatorassociated with each landing Wheel with alternate sectors connectedtothe opposite sides of a constant voltage direct current power source.As each supporting wheel rotates, the commutator -brushes arealternately connected to the opposite sides of the line to produce aconstant amplitude square wave voltage of a frequency proportional tothe rotating speed of the Wheel. This variable frequency current isimpressed upon a speed sensing capacitor which varies the amplitude ofthe square wave in accordance withthe rotating speed 'of 'the wheel.lThis variablefamplitude squarewave is then fed to the .opposite cornersof a bridge rectifier. The rectifier converts the variable-frequencyvariable-amplitude square wave to a nonfpulsating direct current havinga voltage varying with the speed of the wheel. The varying voltagesupplied in 'this manner Lby each wheel energizes separate butlidentical sensing circuits for each wheel. Each sensing circuit isoperable to control acommon bank of relays to release the hydraulicbrake associated with each Wheel in the eventthat the pilot is applyingsu'flicientpressure to cause a slipping or skidding condition to occur,and to retain thebrake released depending upon various factorsgoverningthe safe reapplication of the brake. important `to note thatthe Vpilot has normal control of the brakes up to 'the point at .whichanincipient slipor skid condition develops.

The brakes are of the type normally supplied with meteredpressurizedfluid under the control of the ypilot. The sensing circuits provided bythis invention act to control a three-way brake release valve downstreamof the pilots brake control valve. Thisthree-.way valve closes thepilotspressure control lineto the brake and connects the .brake toableed line Vleading from Vthe brake at each wheel'back to a sump o n thesupply Vside of the hydraulic fluid system .thereby rendering thepilot-actuted braking control ineffective .while the brake release valveis energized,.but restoring thepilot control solong as lthe'bleed valveis de-energized.

Referring to the. schematic wiring diagramfor the Vcontrol Vmechanism asrepresented b y Figures ,la and 1b taken together, `it will :be notedthat the 28 volt direct current supply source is represented at the'left en'd of Figure la. Positive and negative wires '18 and "19,respectively, are connected'tothe main on-ol switch 16 and to a signallamp 17 on the pilots control panel generally indicated by dot-dash box14. The positive or h0t.bus 62 and lthegronnded negative `bus '63.extendfrom supply wires 18 .and 19 to terminals I and iK, respectively, ofterminal strip 38 and to numerous connections in thepor tion of thecircuit shown inFigure lb'. Accordingly, it willbe understo'odthatallparts ofthe system directly connected with positive wire 18 upon the.closing of switch 16'are considered asv hot and are designated 62 Vinthe drawings. Likewise `all leads Vlztavlirlga direct connectionWith-the negative side '19 orfthefpower supply aredesignatedfbynumeral-63. iForlexample, power Jsupply busses 62 and 63extend to positivearid lnegativeiunctons It isv interiorly of thecommutators for each wheel. Inasmuch as terminal strips 38 in eachfigure are identical and serve only as a convenience in tracing a lead-between Figures la and 1b, reference to the strips in the descriptionhas i been omitted for the most part.

The pair of pick-up brushes 50, 51 of the commutator on wheel 10 andbrushes 50', 5,1 on wheel 10 are connected through separate pairs `ofleads 56, A58 and 56', 58 to the opposite input corners of identical dryplate rectifier bridges 5 7, 57. VInterposed in each vof Vleads 58, 58is a speed .sensing capacitor 59, 59 each having a rating of 0.7microfarad, hereinafter abbreviated to nf. The kfunction of capacitors59, 59' is to vary the amplitude of the voltage input to the rectifierbridge in proportion to the rotational speed of its associated wheel.

The commutators on the wheels are thus seen to cooperate with thespeed-sensing capacitors 59, 59 and the rectifier 7bridges 57, 57 toenergize the respective sensing circuits 36 36 with a direct currentvoltage proportional to the speed ,of the wheel to which 'it Visconnected. The initial voltage iimposedon the two principal leads 64,V65 of the sensing circuits' will be 'high due to the high Y speed ofthewheels soon .after the plane touches down on the landing field. 'Theresulting high voltage imposedon the sensing'circuit will `charge thevarious capacitors and venergize each of the relays V, L and W. Two ofthe capacitors .are of particular importance because the energystoredby'them-is utilized upon excessive braking to Veffect release ofthe'brake until the wheel jjhas recovered Yits synchronous speed. 4Thisrelease is accomplished ,by energizing relays controlling a .powercircuit `for a 'brake release solenoid.

This Ybrief summary of the ycontrol system willv serve as afbasis for adetailed description of the components illustrated schematically inFigure 1b.

The sensing or -delectorcrcut 'The identical sensing'or detectorcircuits foreach-wheel are Fshown `in`1-`igure lib as enclosed indottedllinejboxes 36 and 36. For convenience, these circuits'rwill berev ferred to below Yas sensing circuit 36 or 36. A description of uppercircuit 36 will be understood as applying equally to both,'an:d thecorresponding k'Parts of each `will be designated vby the same numeral,or-letter distinguished by va prime. Y u

'The sensing circuit includes Vthree relays V, L and W connected inparallel y.across positive lead 64* and negative lead 65. These leadsyare connected to the output side of bridge rectifier S7 whichenergizesv them with continuons 'direct enrrent'having a voltage varyingin proportion-tothe`rotationalspeed `of-wheel 10. The maximum possiblevoltage is 28-'vo'lts when using a 28 volt power supply, and-variesfrom-this Vvalue to zero under-locked Wheel conditions. Any Atendency"for r frippling resulting v from l-the -operation of the rectifiersk-iscorrected bythe tf. smoothing capacitor 66 connected across the bridgeoutputwt'erminalsin -advance of the sensing relays.

LRielay-'Iais known ,as the Aa1jmirlgrelaysince its activation isrequired to close switch 67 `thereby ar1ning7 the contacts of the lockedwheel control relay V. As will "ebe explained #in d etail presently,relay `V supplies power-to the brake release solenoid control circuitim-V fme'diatelyupon-the-wheel speed -fallingfbelow l0 M. P.,H.iffarmed-bytheL relay. Armingrelay L has Yas many coils as-therearewheels V and'associated sensing circuits, but 'only asingle armaturecommon to all coils and a single'switeh6-7 controlled ,by this armature.Accordingly,*inithepresent-control for two wheels, the arming relay hasone c oilL connected across sensing circuit 36for'wheel'-10,^and asecond coil-Lgacross sensing circuit y"36""for-wheel lil. The singlearmatureis indicated by the -dottedline extending through both coils andthe switch 67/controlled bythe relay. `It will therefore be understoodthatfthelpositionof the-armature and switch isy 'controlled byfthersum1r of fthe variable-voltages applied o to the sensing circuits. Toillustrate, the arming relay is designed to close switch 67 only whenthe combined speeds of both wheels exceeds 20 M. P. H., or if one wheelis rotating at a speed greater than 60 M. P. H. Thus, one wheel may belocked against rotation, but the arming relay switch will remain closedif the other wheel speed is greater than 60 M. P. H.

A 300 af. capacitor 72 is connected across the L relay coil anddischarges through the coil to keep switch 67 closed for a preselectedtime period as, for example, 3.0 seconds following very rapid wheeldeceleration. During normal braking and deceleration, capacitor 72discharges as the wheel speed drops, and relay L opens as the planespeed falls below M. P. H. This allows the brake to remain on in aparked condition with the wheels locked. But during excessive brakingresulting in wheel slip, relay L remains energized to assure a powersupply to the V relay coil for a limited interval after the Wheel speedfalls below 10 M. P. H. At the end of this interval, the V relay isdisarmed but in the interim the slipping condition will have beencorrected by the release control valve and the application of the brakeis placed under the sole control of the pilot through the conventionalmanual brake control facilities. For all combined wheel speeds in excessof l0 M. P. H., switch 67 will remain closed and the locked wheel relayV is armed in readiness to etect release of the brake should the wheelsuddenly become airborne during a bounce condition or strike icy orslippery footing and lock.

Relay V is known as the locked wheel control relay, and is energized byvoltages representing wheel speeds in excess of l0 M. P. H. to open the28 volt D. C. power circuit to the solenoid control relay X. Thecontacts of the V relay are closed when the wheel is locked or slowed toa rotating speed of 10 M. P. H. in order to initiate immediate releaseof the brake until the wheel has reaccelerated or until the skid contactof the W relay has closed. It will be observed that the center contactof relays V and V are connected by a lead 44 which also extends to thecenter contact of switch 67 of the arming relay L. The other contact ofarming switch 67 is connected to the positive bus 62 of the powersupply, while the other contacts of the two V relays are interconnectedby a lead 45 extending to the coil of relay X'. Relay X is activatedwhenever the system is armed and either of the locked wheel controlrelays V or V' is deactivated as a result of the speed of an associatedwheel falling below l0 M. P. H., as will be explained more fully below.

The W relay is of the polarized type and its switch contactor tilts inone direction or the other depending on the direction of current flowthrough its coil. Accordingly, this relay is sensitive to both voltageand the direction of current ow whereas all the others are sensitive tovoltage changes. The W relay is known as the skid control relay and isin a neutral position except when there is a current tiow of a certainmagnitude through its coil. When current Hows downwardly through thecoil, as indicated by arrow R, its recovery contact marked R closes.When the current tlow through the coil is upwardly, as indicated byarrow S, the contact marked S closes.

The direction of current tlow in the W or skid control relay iscontrolled by a pair of capacitors 68 and 69. The recovery ratecapacitor 68 has a capacity of 50 hf., while skid rate capacitor 69 hasa capacity of 500 rtf. and is connected to lead 65 through a resistor70. A rectier or gate means 71 is connected across the two capacitorsand contines the flow of current during discharge and recharging of thecapacitors to desired paths in the W relay circuit as will be explainedmore fully presently. It will be noted that a number of dry platerectiers are provided in the control circuit as a whole and operate toisolate the various time delay circuits from one another as well as toconfine the current flow as desired. The series connected 270 ohmresistor and the l5 nf. capacitor connected across leads 62, 78 at apoint between the W relay contacts are merely for arc suppression.

The power Circuit relays Relays X, X, Y, Z and solenoid valve assembly24 are the components making up the remainder of the automatic controlcircuit for releasing the brake. Each of these components is connectedin the main D. C. power circuit through switch contacts controlled byeach of the sensing circuits and by the pilots manual switch 16. Eachcomponent will be described briey preliminary to a step-by-Stepdescription of the mode of operation under various landing conditions.

The two solenoid control relays X and X" are of similar construction.Relay X is bridged by a 100 uf. capacitor to provide a very brief timedelay of 0.05 second in the opening of the double set of contactscontrolled by this relay. Brief as this interval is, it keeps the Xcontacts closed and the brake solenoid energized during the time oftransfer from skid to recovery of the W relay and during the reversal ofcurrent ow through the W relay. While energized by the cooperativeaction of the sensing circuit relays V, L and W, relay X is effective toconnect the main power bus 62 to leads 74, and energize the brakerelease solenoid 29. Note that the opposite side of solenoid 29 isconnected to negative power bus 63. If the wheel skid is of unusualduration, that is, longer than a predetermined time delay period for theopening of the Y relay, for example 0.4 second, then relay X andassociated parts of the control circuit function to keep the brakesolenoid energized until the wheel speed has recovered to synchronousspeed. It will therefore be understood that the Y relay participates inholding the brake released during skid periods exceeding 0.4 second. Inthe event the skid signal is less than 0.4 second, the departure fromsynchronous speed will be small and the hydraulic pressure is appliedwhen the skid signal ends. The X' relay is also operative to energizethe brake release valve, as during locked wheel conditions when theplane touches down on ice for example, and these will be explained indetail in connection with specific operating situations.

The X relay is conveniently termed a fail-safe solenoid control relayand must be activated by the Z relay to complete the power circuit fromthe hot bus 62, lead 74, lead 75 to the brake release, and solenoid 29to negative bus 63. Consequently, it will be evident that the opening ofthe Z relay contacts will open the power circuit for the X relay anddisrupt the power supply for the solenoid. Since the power supply forthe Z relay coil is opened each time relay X is energized to release thebrake, it is desirable to provide means for holding the Z contactsclosed for a time period in excess of that required for the wheel torecover to synchronous speed. This is done by connecting a 500 af.capacitor across the Z -coil to hold the relay contacts closed and relayX" activated for a time delay period of approximately 3.5 seconds. Thismeans that the brake release solenoid cannot be energized for a longerperiod than 3.5 seconds and provides positive means for deactivating theentire automatic control should one of the relays stick or malfunctionin a manner to keep the brake release solenoid energized.

Relay Y is known as the recovery control relay and is activated onlywhen the solenoid control relay X is deenergized. When X isde-energized, its lower contactor is in the position illustrated andsupplies power from hot bus 62 through lead 76 to the top side of relayY to energize it. Note that the lower side of the Y coil is connected toground bus 63. A 250 nf. capacitor is connected to `discharge throughthe Y relay coil to hold the associated contactor open for a time delayperiod of 0.4 second following the activation of relay X. It X reanna-8.10

mains .activated longer than :0.35 second. this peried plus the .0,05second time delay period vof the X relay will exceed the O .4 seconddelay characteristic of the Y relay. Accordingly, the Y contact willclose thereby rendering the recovery Contact R of .the W relay eifectiveto supply 28 volts D. C. to relay X' to release the brake and allow thewheel to accelerate toward synchronous speed. This function of relay eYis to prevent brake application while the wheel is critically belownon-skid speed as will be explained more fully below.

` I {elay Z is called a fail-safe relay since it operates yto energizepilot `lamp 17 and to retain the normal manual braking system inoperation in the event of malfunctioning of the automatic decelerationlbrake control. `Relay Z YAcan be activated either by way of the oleo orsquat switches 12 and 1 3 located in the vlanding gear struts, or by wayof the upper contacts of relay X. The first of these two current supplycircuits `includes a leadtl conf nected `to .the positive .side 18 ofthe power supply, the two oleo switches 12, 13, a common lead between apair ,of dry plate rectiers 42, 4 3, and leads 37, 77. The second supplyincludes the positive bus 62, vthe upper contact of relay "X', and jlead77. 'In each instance, the lower side of the vZ relay coilis connectedto negative bus 63. Since the two current supply circuits describedabove for the Z relay are connected'to the opposite ends of the mainpower supply switch 16, it will be clearthat, under flight conditions,provision is made for the energization of the Z relay irrespective ofthe position o f switch 16 with the result that'this relay is suppliedwith power irrespective of Whether the pilot remembers to close switch16. Should the plane touch down on the runway, switches 12 and 13 openand relay Z is then dependent on the position of the X relay contactsfor its power supply, as will be explained more fully below.

The Z relay includes a 500 nf. capacitor for holding the relay energizedfor v3.5 seconds thereby maintaining the X" relay energized for thatperiod after X has disrupted the power supply to the Z relay. This delayaction enables Z to maintain the power supply to X for a reasonableperiod of time and then disarms the brake release solenoid power supply.This prevents a sticking .relay or some other .malfunctioning portion ofthe automatic control from energizing the brake release solenoidindefinitely. An indication of failure is shown on `the vpilots panel bythe light 17, which is energized by the upper contact of the'Z relaywheneverthisrelay is de-energized.

Operation in flight Assuming thatthe plane on which the system isinstalled is in Hight, 'the pilot controlled switch 1 6 on panel 14 willbe closed to supply a 28 volt lD. C. current to the hot bus A62. Thisbus Yleads -to a junction interiorly of each commutator and to thecenter contacts of relay X. Another lead 41 supplies power from hot bus62 to yoleo switches 12, v13 associated with each landing Wheel strutand closed `when the plane is airborne. Hence, the closed oleo switchessupply Vpower -through leads 4l, 37 and 7'7 to energize the Z relayclosing its contact downwardly to complete a power circuit from bot bus6 2 through relay X to ground rbus 63, thereby closing the X relaycontacts downwardly. Since the landing wheels are not rotating inighboth Vrelaysare de-energized and the closed oleo switches areeifective to supply vpower .through lead 44, the V relay contacts, andlead 45 to activate relay X', closing the latters contacts downwardly.Current then 4hows through leads 62, 74 and V75, to solenoid 29 and togroundbus' to open the brake release valve and prevent the -applicationofthe brakes owing to the fact Vthat thebrake cylinders126 are ventedthrough conduits 32, 33 and valvemember 28 to sump 33a. lt willtherefore b e recognized that the brakes cannot be applied While theplane is airborne, thus Vproviding a positive safeguard against touchingdown on the runway with the brakes applied Itwill be understood from theforegoing Yexplanation 4that Aduring Vflight relay X remains Venerrrgized and its conftacts are closed downwardly. Under .theseconditions the power supply to the Y relay is broken and its contactremains .closed upwardly. Should a power failure occur for any reason,the failsafe relay "Z will be de-.energ ized (after the 3.5 secondperiod required .to discharge the 500 nf. condenser) and the resulting.upward closing of Vits contact will light pilot lamp 17 .and deactivaterelay Xf thereby .opening .thecircuit for the brake release solenoid 29.The lighting of lamp 17 gives the pilot notice that the automaticdeceleration control system is not functioning vand that the plane must.be lcontrolled bythe fnorf mal or manual control mechanism. It will beunderstood that the automatic portion of the present brake .controlsystem intervenes only if the pilotcontrolled brake vis appliedexcessively hard to skid the Wheel and/ or lock it against rotation, aswill be explained in detail below under specic operating conditions.

The hydraulic brakmg system has only been indicated diagramfrnaticallyinasmuch as the braking system by itself is not a partvof this inventionand it is well known in the prior art. It willbe understood thatpressure purnp 21 is operable to withdraw brake uid from sump 33a anddeliver it under v pressure Vto hydraulic uid pressure tank 22. Conduits32 are provided for delivering this pressurized uid to brake cylinders26 associated with each wheel under the control of the pilot-,operatedbrake .metering valve 23 and of the ysolenoid-controlled three-way valve24. Brake metering valve 23 will be understood as including the .usualmeans such as conduit 23a for returning excess uid back into the systemby way of sump tank 33a'. The Vlatter has a pair of passages 2.7, v27ain ,movable vmember 28, passage 27 being aligned with conduits 32 whenVsolenoid 29V is defenergized, and passage 27a providing a iluidconnection between conduits 32 .and

l33 when the solenoid is .energized thereby venting brake cylinder 26 tosump tank 33a.Y

Landing on dry runway Assuming .that the plane is vin flight ready toland and that the pilots manual control switch 16 is closed to supplypower to the landing wheel commutators and to control relays X', lX" andZ, the operationduring landing on a dry runway is as follows. A dry4runway would normally have a high coefficient of friction.Consequently, excessive application of the brakes bythe pilot wouldcause skidding of one or both wheels. The Squat or oleo switches 12 and13 are'preferably so designed that they open when approximately five toten percent ofthe lfull weight of the plane -is supported by the wheels.So long as the aircraft wheels are off 4the ground or so long as lessthan about ninety percent of the aircraft weight is supported thereby,the-oleo switches will remain closed to supply current to energize the X`relay and the brake releasesolenoid 29 thereby providing a positivesafeguard against touching down on the runway Vwith ther brakes applied.While closed, the oleo switches allow current to flow through rectier42and lead 44 to the closedconf tacts of the V relays from which thecurrent passes by Way of lead 45 to the X relay coil and thence to thenegative bus 63. Once the wheels touch downfthey` pick .up rotationalspeed Very rapidly even though only a part of the aircraft weight issupported b y them. As the Vwheels begin to rotate, the sensing circuits36, 36 are Ienergized and the rotation sensing V relays act to opentheir contacts asV the wheel speed reaches 10M. P. H., therebyinterrupting the current supply tothe X relay and de-energizing brakerelease solenoid 29. It will be understood Vthat this de-energization ofthe brake solenoid occurs regardless of the degree to which .thecraftweig'ht is k supported by the landing gear and solely in responseto thespeedpf Atl:1e.wl 1eels as sensedby the respective V relays. I fthe plane bounceswith the brakes applied. the Vtfrelays will.bemeienenrfgized .t0 .restare the current flow to relay X' and effectrelease of the brakes by energizing solenoid 29 in the manner describedabove until such time as the wheels again touch down and reach arotating speed of 10 M. P. H. or greater. Upon being energized, solenoid29 is operative to shift valve 28 in opposition to spring 31 to theposition illustrated in Figure la wherein brake cylinder 26 is vented tothe sump tank 33a by way of conduits 32, 33 and the horizontal passagein valve 28.

The specific manner in which the various components of the decelerationcontrol of this invention function during the initial touch down andsubsequent phases of the landing operation is as follows. Owing to thehigh surface coeflicient of friction for dry runway conditions, thewheels accelerate in an extremely brief interval and usually before theoleo switches open. This wheel rotation causes commutatore 49, 49' incooperation with capacitors 59, 59' to supply a square wave voltage ofvariable frequency `and amplitude to bridge rectifiers'57, 57. These inturn energize the sensing circuits with a direct current voltageproportional to the speed of the associated wheel. Since the operationof the sensing circuits are identical and controlled by the signalreceived from their respective wheels, only the operation of circuit 36for wheel 10 will be described, it being understood that the Sameexplanation applies to circuit 36'. As soon as the wheels rotate abovel0 M. P. H., the locked wheel relay V is energized to open itspreviously closed contacter. At substantially the same instant, armingrelay L is energized to close switch 67 to arm the center contact of theV relay by connecting it to the positive bus 62. The recovery ratesensing capacitor 68 and the skid rate sensing capacitor 69 connected inparallel with the W relay coil and across sensing circuit power leads 64and 65 become charged to a voltage proportional to the wheel speed.

Let it be assumed that the brake is applied excessively to wheel 1t) bythe pilot opening brake fluid metering valve 23 to allow pressurizedhydraulic fluid to ow from a supply tank 22 to brake cylinder 26, thisflow taking place through duct 32 and a passage 27 in valve 28controlling fiow therein. Normally, brake release solenoid 29 isde-energized, and spring 31 is effective to hold valve 28 to the left ofthe position illustrated so that passage 27 is aligned with duct 32.

The high coefficient of friction prevents as rapid deceleration of thewheel as would occur on a slippery runway, but the undesirably highdeceleration rate following excessive brake application causes animmediate and proportional drop in the voltage applied across leads 64,6.5 of the sensing circuit. The charge present on capacitors 68 and 69as deceleration of the wheels starts represents stored electrical energywhich begins to discharge into the sensing circuit by way of lead 64,the V relay, lead 65, then in parallel through resistor 70 and the coilof the W relay and rectier 71. This upward flow of current in the Wrelay coil closes skid contact S and completes a power circuit from bus62, the relay contactor, and through leads 79, 78, 45 to the coil ofrelay X thence to grounded bus 63. The activation of relay X closes itscontacts downwardly to supply power from bus 62 to leads 74, 75,solenoid 29 and ground bus 63, thereby shifting valve 28 to the positionshown in Figure la wherein the braking fluid can escape from brakechamber 26 to sump 33a by way of conduit 32, 33 and valve 28. Therelease of the brake allows the wheel to reaccelerate to synchronousspeed very quickly and before capacitor 69 becomes fully discharged. Thereacceleration restores the voltage level in the sensing circuit andrecharges the capacitors. As this recharging begins, the dischargecurrent flow through the W relay ceases and the skid contact S opens tode-energize relay X', allowing spring 31 to shift the brake releasevalve to the left and restore the brake to manual control. A short timedelay is involved before the brake can be reapplied due t0 the timerequired for the brake pressure to build up.

During this period, capacitors 68 and 69 are recharging in a reversedirection to that described for the discharge phase of the cycle.

ln recharging, the main current ow to the larger capacitor 69 isdownwardly through resistor 70 rather than through the relay coil byreason of the blocking action provided by rectifier 71. The chargingcurrent for the recovery rate capacitor 68 does flow downwardly throughthe relay and may be strong enough to close the contacter to the Rcontact. However, should this occur it will have no effect on relay Xbecause the skid cycle for dry runway conditions is always shorter thanthe time delay period of 0.4 second required for the Y relay contact toclose and render the R contact effective to again close the brakerelease circuit. But this is immaterial since the wheel recovers tosynchronous speed very quickly on a dry runway.

On dry runways the pilot may sense the effectiveness of the braking andcontrol the application of the brakes accordingly. If the brakes areproperly applied, there is no sudden voltage decay in the sensingcircuit in which event the skid rate sensing capacitor is not calledupon to supply stored energy to effect closing of the skid contact S toenergize relay X and the brake release solenoid. But when the sum of thewheel speed falls below 20 M. P. H. representing a plane speed of l0 M.P. H., the arming relay L opens to disarm the V relay contacts andreturn the control of the brakes solely to the pilot and the manualcontrol system. The pilot thus has parking brakes despite the closing ofthe V relay contact since the power supply to this contact has beendisconnected by the arming relay.

Landing on a wet or icy runway Let it next be assumed that the plane islanding on a wet or icy runway having a low coefficient of friction. If,under these circumstances, the brakes are applied excessively as theplane touches down, or is traveling at high speed along the runway, thewheels will decelerate very quickly as the brakes are applied and willlikely lock instead of merely decelerating to a speed considerably belowthe linear speed of the plane as they do on a dry runway. Furthermore,unlike dry runway landing conditions, one wheel may lock or go to zerospeed while the other may decelerate much more normally. In eitherevent, the excessive and extremely rapid deceleration or locking ofwheel 10, for example, results in the complete collapse of the voltagesignal to sensing circuit 36. This causes the locked wheel relay V tode-energize and close its Contact thereby supplying power from bus 62through arming switch 67 of the L relay to lead 45 and energize relay Xand brake release solenoid 29 to release the brake on the locked wheel.The L relay is closed despite the locking of wheel 10 because of thedelaying action of capacitor 72 on this relay. The relay would alsoremain closed if wheel 10 is operating at a speed of 60 M. P. H. ormore. Upon release of the brake for wheel 10, the skid rate and recoveryrate capacitors begin discharging stored energy into the sensing circuitas described above, the discharge current of capaictorr69 flowingupwardly through coil W to close skid contact S and continue the releaseof the brake despite the opening of the V relay contact as the wheelaccelerates above l0 M. P. H.

During the initial phase of the reacceleration phase, the wheel may beonly partially up to synchronous speed as the rising voltage signal fromthe commutator equals the falling voltage of discharging capacitor 69.At this instant there is no current ow in the W coil and the S contactwill open to de-energize relay X. However, the time delay constant of0.05 second of the X relay provided by its uf. capacitor is suficient tokeep the brake release solenoid energized until the charging current forrecovery capacitor 68 begins flowing downwardly through coil W to closerecovery contact R. This contact remains closed while the wheelcontinues to recover speed anual@ to-,recharge .the capacitors 6 8 and69, it being recalled fromthe discussion of dry runway operations thatduring recharging, the recharging current for 68 is restricted byrectier 71 to flow downwardly through coil W to hold the brake releaseduntil .the recovery rate sensing capacitor becomes charged. Ascapacitor68 becomes charged, the recharging current flow ceases and contact R,opens to .deactivate the brake release solenoid.

The importance of the roles played by the two sensing capacitors willvbe appreciated from the fact that, during short skid periods, the skidsensing capacitor is .effective V to -release the brake untilsynchronous speed is reached even though the wheel should lock, butduring long skid periods the recovery sensing capacitor takes over forthe latter portion of the recovery period and is effective to continueAthe' brake release until synchronous speed is achieved. If a wheel hasonly recovered to substantially synchronous speed when the capacitorscease discharging, it would obviouslybe vimprudent to permitreapplication of the brake. But with the arrangement provided here, therecovery rate capacitor 68 acts in the manner described to ,keep vthebrake released until vthe wheel vhas recovered to synchronous speed.

lf subsequent brake applications do not cause excessive .wheeldeceleration, the skid cycle operation may well be of sufcient durationto permit recovery of wheel speed. If this occurs within 0.4 second,relay Y will remain energized Vand render the R contact of the W relayineiective to prolong the brake release since there is no necessity forsul ,release he operation of the arming relay and locked wheel controlrelay V during the nal phase of the landing operation is the same asdescribed above under dryY runway operating conditions.

Another important capability of the present control is that of effectingrelease of the brakes immediately following touch down on ice or on anunusually slippery runway. In such cases the wheels undoubtedly will notaccelerate to any appreciable extent before brake application with theresult that the sensing circuits are only partially energized. But thispartial energization su'ices to actuate Vthe armingrelay to .close thepower supply to the lockedwheel Vcontrol relays V and V. By virtue `ofcapacitors 72, 72 in ccircuit ,with Ithe arming relay coils, the Vrelays vare kept -armed for a period up to 3.0 seconds after the wheel.orV wheels -lock to ykeep Ythe brakes released until the wheels havehad an opportunity to reach synchronous speed. `It will therefore beappreciated that the V relays perform a very important function underthese particular operating .endings- From the foregoing it will beunderstood that it is irn- ,material to the operation of the brakecontrol whether one yor both Ywheels are skidding on the slipperyrunway. Thus, both of the sensing lcirnits are -cormected inparallel theX' relay and `other power Y circuit control relays, and ,each sensing vcircuitis operable to ,control-the brake associated Zervvith in such aWay .2S to render the ,pilot control ineffective `so long fas eitherwheel is skidding or slipping at an vundesirable rate.

'.It isto be understood that the ratings yof the resistors Iandcapaictors, the drop `out speeds of the V and L relays, ,the voltages.employed, etc., specified above by way of example 'may be varied toobtain diiierent .operational periods for the components withoutdeparting from kthe Aprinciples or spiritof this invention.

' While the particular Vbrake control ,system herein shown and,described in ,detail isfully .capable of attainingthe Obie'ts @dPmVdPg. th .advantages hereinbcfore stated, v,it lis to .be `understoodV`that it is merely illustrative .of .the presently preferred embodimentof the inyentionand that no v1irnitatirms are A intended to the detailsof ,construction pr designherein shown other .than as defined in theappended- Claim- "Islam l. An automatic whe el .brake control system of,the

type having an automatic brake release control for overriding a manualbrake control upon .excessive application of the brake, said systemincluding manually controllabile brake means, release means operable ltorelease said .brake means independently of the manual control therefor,and electrical means actuatable by the rotation of the braked wheel foractuating said release means, said electrical'means including a variablevoltage signal generator adapted to be driven by said wheel, va sensingcircuit connected to be energized by said generator and including meansfor storingl electrical energy from said generator, means in saidsensing circuit for sensing a locked `wheel condition and connected tosaid brake release means and operable `to release said brake in responseto the locking of said wheel, and means operable by stored energyderived from said energy storing means to hold s aid brake releasedwhile said wheel is regaining `speed and while the energy drained fromsaid energy storage means is being replenished.

Y 2. An automaticwheel brake control system of the type having anautomatic brake release control for overriding a manual brake controlupon excessive application ofthe brake, said system including manuallycontrollable brake means, release means operable to release said brakemeans independently of the manual control therefor, and electrical meansactuatable by the rotation of the braked wheel for actuating saidrelease means, said electrical means including Aa variable voltagesignal generator adapted to be driven by said wheel, a sensing circuitconnected to be energized by said generator and including a pair ofelectrical energy storing means of different capacity arranged to becharged from said generator, a polarized relay in circuit with saidenergy storing means, said energy storing means being operable toenergize said relay selectively in different directions depending uponthe direction of current ow through said relay from said energy storingmeans, means connected to said energy storing means and to said relayfor determining Athe discharge of said venergy storing means throughsaid relaydepending upon the direction of current ow therethrough, andmeans including a power circuit connected between said relay and saidbrake release means operable to activate said brake release and releasethe brake when current ows through said relay in one direction from saidenergy storing means and operable When current flows through said relayin the opposite direction from a portion only of said energy storingmeans to maintain said brake released for a substantially longer periodnor.- mally adequate for said wheel'to reaccelerate approximately tosynchronous speed.

3. In an automatic wheel brake control system, the combination inelectrical circuitry; of a sensing circuit coupled to direct currentcommutating means Yrotating with a wheel to be braked including meansfor translating the output of said commutating means to a voltage signalproportional to wheel speed, a power circuit including brake releasemeans, control means in said power circuit for activating said brakerelease including a polarized relay in said sensing circuit, a pair ofenergy storage means of different capacity in said sensing circuitadapted to be charged from said commutating means and including meansfor determining the discharge of said energy storage means depending onthe direction of current flow through said polarized relay to activatesaid brake. release, both of said energy storage means being effectiveto activate said brake release` depending upon the direction of currentow, only one of said energy storage means being effective to continuethe period of brake release, said continuation of brake release beingdependent upon the rate of wheel speed recovery following release of thebrake.

4. A wheel brake control system as dened in claim 3 characterized in theprovision of manual means for operating the Wheel brake, rst and secondrelay means in said power circuit, said rst relay means being connected13 with said polarized relay and operable to arm the power circuit andto deenergize said second relay means, said second relay meansdeenergiring said power circuit after a predetermined period, wherebythe effectiveness of said manual brake control means is restorednotwithstanding the operating condition of said commutating means and ofthe sensing circuit energized thereby.

5. In an automatic wheel brake control system having a ground contactingwheel adapted to drive a direct current commutating means to produce avariable frequency square wave, means for converting said square wave todirect current having a voltage proportional to Wheel speed, a manualbrake control for braking said wheel, an automatic brake release fordeactivating said manual control during excessive braking, said brakerelease includng a normally open power circuit, a sensing circuitconnected to and energized by said variable voltage direct current andincluding energy storing means arranged to be charged with energy duringperiods of wheel acceleration, and relay means in said power circuitactivated by said stored energy during periods of excessive wheelbraking and operable to activate said brake release to permit said Wheelto reaccelerate.

6. An automatic wheel brake control system as defined in claimcharacterized in that said square wave converter means includes asemi-conductor bridge connected across s aid commutating means through acapacitor and cooperating therewith to convert said variable frequencydirect current square wave into direct current having a voltageproportional to Wheel speed.

7. In an automatic brake control system for use in over-riding a manualbrake control instantly that overbraking results in excessivedeceleration of the braked wheel, said system being of the typeincluding a manually operated brake control for a wheel, brake releasemeans for deactivating said manual control, a speed sensing circuitconnected to and operable to activate said brake release means when thespeed of the wheel falls sufficiently abruptly to indicate over-brakingby said manual brake control; that improvement in said brake controlsystem comprising means for energizing said sensing circuit with avoltage which varies proportionally to Wheel speed and including incombination direct current commutating means driven by the braked wheeland producing a square wave direct current having a frequency varyingwith wheel speed, a rectifier bridge connected across said commutatingmeans through a capacitor and cooperable therewith to convert thecommutator output to direct current having a voltage proportional towheel speed, and means connecting the output of said rectifier bridge tosaid sensing circuit to energize the same.

8. An automatic brake control system as defined in claim 7 characterizedin that said system includes a plurality of wheels arranged to bemanually braked in unison by said manual brake control, each of saidwheels having an associated speed sensing circuit and commutating andconverter means for energizing its speed sensing circuit with directcurrent voltage proportional to wheel speed, and common power supplymeans interconnecting each of said sensing circuits with said brakerelease means and, whereby said brake release means is activated throughsaid power supply means when the rate of voltage change in any one ofsaid sensing circuits indicates over-braking of the wheel associatedwith that speed sensing circuit.

9. An automatic brake control system as defined in claim 8 characterizedin that each of said speed sensing circuits includes a pair of energystoring means in combination with a polarized relay and a semi-conductorso connected as to activate said relay in one direction to activate saidbrake release means when the voltage across said sensing circuit fallsat an excessive rate and also operative to close in the oppositedirection to maintain said brake release activated for a briefadditional period measured by the period required substantially torestore the energy level of one of said energy storing means.

l0. In an automatic brake control system for a plurality of wheels, thecombination including separate speed sensing circuits adapted to becoupled to direct current commutating means -driven by each wheel to bebraked and including means for energizing each sensing circuit withdirect current having a voltage proportional to the speed of theassociated wheel, a manual brake control for said wheels, over-ridingbrake release means and a power circuit for activating said brakerelease means, each of said sensing circuits including a locked wheelrelay, an arming relay common to all of said circuits operable to armsaid brake release power circuit when the sum of the wheel speedsexceeds a predetermined value, a polarized relay including a pair ofenergy storing means in each of said sensing circuits, one of saidenergy storing means being operable to discharge through said polarizedrelay in response to an excessive decrease in the associated wheel speedto close said polarized relay thereby activating said brake release, theother of said energy storing means including gate means cooperabletherewith to activate said polarized relay in the opposite direction asthe excessively vbraked wheel regains speed and being operable tomaintain said brake release activated while said braked wheel recoverssubstantially to synchronous speed.

11. In combination with a manually controlled wheel braking system for aplurality of wheels, an automatic brake release operable to deactivatesaid braking system temporarily independently of said manual brakingsystem, said automatic release means including an electrical sensingcircuit for each wheel and separate signal generating means driven byeach wheel being braked connected to energize a particular one of saidsensing circuits with a signal voltage proportional to wheel speed, apower circuit for said brake release, arming relay means connected incircuit with each of said sensing circuits and operable when the sum ofthe wheel speeds exceeds a predetermined value to arm said power circuitfor said ybrake release means and to disarm said power supply at loweraggregate wheel speeds, circuit breaker means in said power circuit, anddecelerating wheel speed sensing means in said sensing circuitresponsive to changing signal conditions in said sensing circuit andoperable to close said circuit breaker means to release said manualbrake in response to abnormally rapid Wheel deceleration indicative ofexcessive manual brake application.

l2. In an automatic wheel brake control system for a plurality ofaircraft wheels, the combination comprising normally deactivated brakerelease means for said wheels, a power circuit, means responsive torotating wheel conditions of each wheel to generate independent signals,separate locked wheel relay means responsive one to each of said signalsand operative to close said power circuit to energize said brake releasemeans at or near locked Wheel conditions, means for storing said signalenergy, relay means activated by said stored energy to operate saidbrake release means through said power circuit, and an arming relay insaid signal circuit operable when the sum of the individual wheel speedsexceeds a predetermined value to arm the power circuit for said brakerelease means and operable to disarm said power circuit at aggregatewheel speeds below this predetermined value, whereby said brake controlsystem is operable to release the brake on a wheel which locks beforesaid wheel has accelerated to synchronous speed due to touching down ona slippery runway.

13. A brake control system as defined in claim 12 characterized in thatsaid relay means is polarized, and in circuit with said relay means is asecond means for storing signal energy simultaneously with said firstmentioned energy storing means, rectier means in circuit with saidsecond energy storage means and relay means gperabl t0 utilizevre-hargi-rlg of Said lSecond energy storing means to maintain saidarmed brake release power circuit activated during Va period of Wheelspeed recovery following .braked wheel .braking until said locked wheelhas reaccelerated substantially to Isynchronous speed.

14. In an automatic wheel brake control system for a plurality ofwheels, the combination co-mprising normally deactivated brake releasemeans, a power circuit for said brake releasevmeans having an armingswitch therein, means responsive to rotating Wheel conditions of eachwheel to generate separate brake release signals, separate relay`circuits connected to and energized by signals from an associated oneof said wheels, said relay circuits each including a locked lwheel relayand an associated .energy storage means, said wheel generated signalsbeing effective to charge said energy storage means and to Aactivatesaid locked wheel relay to maintain said power circuit switch closed atwheel speeds near locked wheel conditions and open at higher wheelspeeds, an arming relay coil common to all of said relay circuits andincluding means for holding said arming relay coil energized for aV timeinterval after the predetermined aggregate speed of said wheels fallsbelow the Speed required to energize the arming relay, time delay meansoperable to close said arming switch so long as the sum of the wheelspeeds exceeds a predetermined speed higher than the speed at Whichsaidlocked wheel relays open, deceleration-sensitive Arelay means includingsaid energystoring means in each `of said relay circuits ,operable toclose said armed power circuit to activate lsaid brake release upondeceleration of an associated one of said wheels due to excessivebraking, said decelerationfrsensitive relay utilizing energy derivedfrom the last mentioned energy storing means.`

l5. A brake control Y system as defined Vin claim 14 characterized inthat said time delay means for `said arming relay is operable tomaintain said arming relay energized to arm said power circuit for aperiod not substantially in excess of three seconds.

16. In an automatic brake control system for a plurality of wheels, thecombination including sepanaspeed .sensing ,circuits adapted to becoupled `,to direct current commutating means driven by each wheel to bebraked and including means for energizing each sensing circuit withdirect current having a voltage proportional to the speed of theassociated wheel; a manual brake control for said wheels having normally,deactivated brake release means; said speed sensing circuits eachincluding locked wheel control relay means, .an arming relay common to`all speed sensing circuits for arming said brake release mean-s, andpolarized relay means including means operatively connecting the same to,activate said brake release means, a plurality of sensing meansconnected to activate said polarized relay means in one direction inresponse to over-braking of the associated wheel, a rectifier in circuitwith said sensing means in such manner that only one of said pluralityYof sensing means activates said polarized relay in the oppositedirection following an increase in the volatge signal to' said speedsensing circuit after the activation of said brake Vrelease V,to`maintain the brake release activated for a brief period while thereleased wheel is recovering substantially to synchronous speed.

17. An automatic brake control as defined in claim 16 characterized inthat 4said arming relay includes a plurality of coils one of which is ina different one of said sensing circuits and 'having an armatureoperated arming switch common t0 all of said coils, said arming .relaybeing `responsive to the sum Vof the wheel speeds to maintain'saidswitch closed so long as the sum of the wheel speeds exceeds apredetermined rate.

Y References Cited in the le of this patent Y UNITED STATES PATENTS2,256,287 McCune Sept. 16, 1941 2,663,521 Yarber Dec. 22, 1953 2,753,017Curl et al. Iuly 3, 195,6 2,788,186 Wilson Apr. 9, V19,57 V2,799,462Steigerwald July 16, 1957

